In conventional paper dryers of the prior art an upper felt is guided around the upper dryer rolls by a felt roll and a lower felt is guided around the lower dryer rolls by another felt roll. In the course of the drying operation the paper is sandwiched between the felts and the dryer rolls but is unsupported in the region in which it passes from a lower dryer roll to an upper dryer roll and vice versa. While such a machine provides a most efficient drying operation it is not capable of as high speed operation as is desirable owing to the presence of the "draws" or regions in which the paper web is unsupported as it passes between the upper and lower dryer rolls.
In order to afford a higher speed operation than is possible with a conventional dryer, there has been developed what is known as "unifelt" or "serpentine" dryer units in which a single endless felt is trained around upper and lower dryer rolls and associated guide rolls. Dryer sections of this type are capable of operating at speeds in excess of 2,000 feet a minute. A number of such sections are arranged immediately following the discharge of the press section of the paper machine after which the paper web has acquired sufficient strength to permit a return to the conventional dryer system.
The serpentine or unifelt run dryer has the advantage of being capable of operating at higher speeds at the wet end of the machine than does the conventional dryer for that the former supports the web over substantially its entire length. While this is true it does not provide as efficient a drying as does a conventional dryer.
In use of the serpentine dryer unit, the paper web is dropped from the last press roll to a suction roll and then it is transferred to the first bottom dryer. In the transfer region between the press and the first dryer the paper web runs outside of and below the felt. In this transfer section, which is approximately 5 to 10 feet long, the paper web, which could be about 300 inches wide, tends to delaminate from the fabric and begins to droop. This phenomenon is most pronounced at the edges of the section. As a result of this drooping, when the paper web hits the dryer section it gets crushed or folded over, thus forming a defect. As the speed increases the whole sheet begins to delaminate, wrinkles form, and breaks occur.